1. Field of the Invention
The present invention relates to an antenna which is suitable for high-quality wireless communications in the microwave and extremely high frequency ranges, where communications are performed while switching the rotation direction of a circularly polarized wave and a maximum gain direction of radiation directivity.
2. Description of the Related Art
In recent years, there are increasing needs for rapid large-capacity communications in a closed space, e.g., an indoor space, as exemplified by indoor wireless LAN, for example. In a closed space such as an indoor space, there are not only direct waves along a line-of-sight between antennas, but also delayed waves due to reflections from the walls, ceiling, or the like exist, thus constituting an environment of multipath propagation. This multipath propagation is a cause for deterioration of the communication quality.
In order to suppress deteriorations in communication quality that are caused by delayed waves in a multipath propagation environment, one method employs an antenna which permits switching of a maximum gain direction of radiation directivity. This is a method that enhances the communication quality by switching the maximum gain direction of the antenna and performing transmission/reception in a selected optimum state.
There is also a method which employs a circular polarization antenna in order to suppress deteriorations in communication quality caused by delayed waves in a multipath propagation environment. A circularly polarized wave is an electromagnetic wave which advances while the direction of its electric field vector rotates with time. When the direction of advancement is viewed from a fixed place, a circularly polarized wave whose electric field vector rotates clockwise is referred to as a clockwise circularly polarized wave, whereas a circularly polarized wave whose electric field vector rotates counterclockwise is referred to as a counterclockwise circularly polarized wave.
Usually, it is difficult to generate a completely circularly polarized wave, because it will merge with a polarization component of the opposite rotation, thus resulting in an elliptically polarized wave. The ratio between the major axis and the minor axis of this ellipse is referred to as an axial ratio, which serves as an index representing the characteristics of the circularly polarized wave. The smaller the axial ratio is, the better the circular polarization characteristics are. In a usual circular polarization antenna, the value of the axial ratio is 3 dB or less.
An antenna which is designed to transmit or receive clockwise circularly polarized waves cannot transmit or receive counterclockwise circularly polarized waves. Similarly, an antenna which is designed to transmit or receive counterclockwise circularly polarized waves cannot transmit or receive clockwise circularly polarized waves. Generally speaking, a circularly polarized wave which has impinged on an obstacle such as a wall becomes a circularly polarized wave of the opposite rotation, and is reflected therefrom. In other words, through one reflection, a clockwise circularly polarized wave becomes a counterclockwise circularly polarized wave, and through another reflection, again becomes a clockwise circularly polarized wave. Therefore, by using a circularly polarized wave for indoor communications, multipath components ascribable to a single reflection can be suppressed.
As a planar antenna which is capable of transmitting and receiving circularly polarized waves, a planar antenna that is described in Ramash Garg et al., “Microstrip Antenna Design Handbook”, Artech House, p. 493-515 (Hereinafter, Non-Patent Document 1) is well known, for example. FIG. 15A is a schematic illustration showing a generic linear polarization antenna, and FIGS. 15B and 15C are schematic illustrations showing the generic circular polarization antenna structures described in Non-Patent Document 1. In order to generate a circularly polarized wave, it is necessary to employ two linear polarization components which have orthogonal planes of polarization and whose phases are shifted by 90°. In a commonly-employed radiation conductor plate 31 as shown in FIG. 15A, which is shaped so as to be axisymmetrical with respect to a line extending through a center of gravity 32 of the radiation conductor plate and a feed point, resonation occurs only in such a manner that the electric current oscillates in the direction of the aforementioned line, whereby a linearly polarized wave having a plane of polarization in this oscillation direction results.
In order to generate a circularly polarized wave from the aforementioned axisymmetrically-shaped radiation conductor plate 31, the aforementioned resonation must be separated into two orthogonal resonations. In order to separate the aforementioned resonation, the structural symmetry of the radiation conductor plate 31 may be broken as shown in FIGS. 15B and 15C, for example. At this time, depending on where the symmetry is broken, a counterclockwise circularly polarized wave may be excited as shown in FIG. 15B, or a clockwise circularly polarized wave may be excited as shown in FIG. 15C.
However, as an antenna to be internalized in a laptop computer or an antenna for a mobile device, circular polarization antennas such as those shown in FIGS. 15B and 15C are unsuitable. The position and orientation of such a mobile terminal may greatly change, so that a circular polarization antenna having a fixed rotation direction may not be able to perform transmission/reception when it is reversed in orientation, for example. Therefore, as an antenna for realizing high-quality and high-efficiency communications in a mobile terminal device, there is needed an antenna that permits control of the rotation direction of a circularly polarized wave.
Moreover, communications with an even higher quality and higher efficiency can be realized by simultaneously realizing the aforementioned two functions that are effective for elimination of multipaths, i.e., a “function of switching the maximum gain direction of radiation directivity” and a “function of switching the rotation direction of a circularly polarized wave”.
One conventional antenna that simultaneously realizes the aforementioned two functions, i.e., “switching of the rotation direction of a circularly polarized wave” and “switching of a maximum gain direction of radiation directivity” is a phased array antenna whose array elements are antennas capable of switching circular polarization (see Japanese Laid-Open Patent Publication No. 2000-223927 (Hereinafter, Patent Document 1)). FIG. 16A is a block diagram showing the construction of one unit of a conventional circular polarization switching type-phased array antenna described in Patent Document 1, supra. FIG. 16B is a block diagram showing the overall construction of a circular polarization switching type-phased array antenna.
As shown in FIG. 16A, in each antenna unit of a conventional circular polarization switching type-phased array antenna, switching of the rotation direction of a circularly polarized wave is realized through control of external signals s41 and s42, and switching of the radiation phase of the antenna is realized through control of external signals s43, s44 and s45. By building a multi-element construction composed of such units, as shown in FIG. 16B, and controlling all external signals by using an external controller, switching of the rotation direction of a circularly polarized wave and a maximum gain direction of radiation directivity of the entire phased array antenna is simultaneously realized.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2000-223927    [Patent Document 2] Japanese Laid-Open Patent Publication No. 9-307350    [Non-Patent Document 1] Ramash Garg et al., “Microstrip Antenna Design Handbook”, Artech House, p. 493-515
However, an antenna having the above-described conventional construction is unsuitable as an antenna for a small-sized device or terminal because of problems such as: a plurality of phase shifters being required, thus resulting in complicated construction and control, and switching of a plurality of feed lines being required, thus resulting in a large insertion loss associated with switching elements.